Solders are low melting metal alloys which can be melted to join other metals having higher melting points. The technology of such materials, which are usually low melting alloys of tin, lead, bismuth, silver and similar metals, has been developed over a period of several hundred years. Nevertheless, even though the basic technology is much older, the last thirty years, which many consider to be the dawn of the electronic age, have produced many significant advances.
A significant area of improvement in solder paste technology involves the use of organic acids as fluxing agents. A 1949 patent (U.S. Pat. No. 2,470,957) issued to J. E. Strader discusses the use of glutamic acid hydrochloride as a solder flux constituent. Other organic acids used as solder fluxes include lactic, oleic, stearic, glutamic and phthalic acids (see "Solders and Soldering" by Howard H. Manko, McGraw-Hill Book Co., 1964). Many of these acids are successfully used in applications where the residues left after the soldering operation can be washed away using water as a solvent. Hence they have been widely promoted as water-soluble flux systems.
Although water-soluble organic acids can give more active fluxing action than the so-called water-white rosin fluxes, they sometimes are too corrosive for delicate soldering applications. Because of the potential corrosiveness of organic acids, many patents have issued related to more complex organic esters, alcohols and amines, which retain a high level of flux activity without having the corrosiveness of their acid counterparts. Examples of such organic fluxes are those disclosed in U.S. Pat. Nos. 4,180,616; 4,092,182; 3,944,123; 3,675,307 and 3,099,590. The fluxes disclosed in U.S. Pat. No. 4,180,616 are derivatives of bile acids, for example cholic acid. These acids are not as chemically reactive as the shorter chain aliphatic acids, and are therefore not as corrosive in soldering applications.
The corrosiveness of a solder flux in a given application is strongly dependent on the amount of the particular flux present, as well as the chemical nature of the other organic chemicals present. Therefore, the corrosiveness of short chain aliphatic acids (e.g., succinic acid) may be fairly high if the concentration is high and if no other organic chemcials are present to neutralize the acidity during the soldering operation. However, if a low concentration of succinic acid is used as a component of an otherwise unactivated flux system, the overall corrosiveness is low.
When organic acids are incorporated into water soluble flux systems, relatively high concentrations of the acids are generally used. Upon completion of the soldering operation, these acids and their residues must be cleaned off by washing with solvents. However, many microelectronic applications cannot tolerate water cleanup after the soldering operation. Therefore, it would be advantageous to find a way to use organic acids efficiently in systems requiring organic solvent cleanup.
The amount of organic acids used in typical solder fluxes disclosed in the prior art varies greatly, but the total amount of organic acids is generally greater than 1% by weight. It has been found however, that organic acid concentrations higher than 1% lead to difficulty in cleaning the flux residues after soldering, particularly when organic solvent cleaners are used. For example, U.S. Pat. No. 2,898,255 discloses the use of organic acids such as oxalic, malonic, succinic, glutamic and adipic acids in rosin-based solder fluxes. The amount of dicarboxylic acid needed to give satisfactory performance is claimed to be 1-13.5% by weight.
U.K. Pat. No. 1,458,351 discusses use of unsaturated C.sub.12 -C.sub.36 aliphatic monocarboxylic acids in conjunction with rosin. It also mentions fluxes containing one or more aliphatic or alicyclic dicarboxylic acids such as succinic, azelaic, sebacic, fumaric and hydrophthalic acid. However, the maximum and minimum amounts of acids that are acceptable are not disclosed.
Other commonly known flux systems are organic esters, amines, alcohols, phosphates and long chain aliphatic monocarboxylic acids, e.g., stearic acid and derivatives of such materials.
One of the problems with the prior art fluxes is that they are difficult to use in that they must often be used in excessively high concentrations to perform their function of effectively cleaning the metal surfaces to be joined. As mentioned above, these materials must be cleaned off the solder assemblage. On the other hand, when small concentrations of flux are used, insufficient surface cleaning results and a poor solder joint is obtained. This is frequently indicated by the occurrence of low metal retention and solder balling.
The term "solder balling" refers to the undesirable tendency of a solder paste, when heated during reflow, to form small spheres of solder instead of forming a single solder fillet. Solder balling is primarily caused by incomplete removal of surface oxides during the early stages of the reflow process. These oxides retard or prohibit the coalescence of the solder when heated to the melting temperature. The occurrence of solder balling directly affects the amount of solder alloy present in the metal joint. The term "metal retention" is commonly used to express the comparison between the theoretical amount of solder alloy in the solder paste and the amount of solder actually deposited at the metal joint. Metal retention is expressed in percent and is calculated by the following equation: ##EQU1## Metal retention values greater than 85% are generally acceptable, and values approaching 100% are ideal.